1. Field of the Invention
The present invention relates in general to a high-speed and low power consuming operational amplifier employed, for instance, for driving a circuit having a high input capacity such as an analogue to digital (referred to as "A/D") converter or for fast driving a circuit having a low input impedance and, in particular, to those effective to applications which require of extremely low-power consuming operations among general purpose operational amplifiers.
2. Brief Description of the Related Art
Trends toward the low power dissipation, the high operational speed and the miniaturization of the components have been accelerated in response to the recent demands on thinning and down-sizing of equipment and to the technological progression. In the operational amplifiers which act as a sort of arithmetic circuits, the low power consumption and the extremely high operational speed have been achieved by means of a low supply voltage, an unbalanced power supply and a high density integration.
Constitutions shown in FIGS. 6 and 7 have been known up to now as conventional output stages of the operational amplifiers. FIGS. 6 and 7 are block diagrams for showing constitutions of the output stages of the operational amplifiers particularly in detail. FIG. 6 illustrates a circuit which is constituted of bipolar transistors while FIG. 7 illustrates the circuit which is constituted of metal-oxide-semiconductor (referred to as "MOS") transistors. In FIGS. 6 and 7, a numeric character 1 stands for an operational amplifying circuit except for a high drivability output stage, 5 stands for a normal input signal terminal and 6 stands for an inverted input signal terminal. Output signals produced from the operational amplifier 1 are applied to of the output stage wherein the signals are transformed in impedance to be applied to an output signal terminal 7. In either case, an emitter follower configuration or a source follower connection is biased with a constant current circuit 100 or 101 so that an idling current is kept constant. Such constitution of the output stage as mentioned above requires to allot large transistors in size to final stage transistors Q6, Q12, M6 and M12 in order to drive heavy loads which is connected to the output terminal 7.
However, because aforesaid conventional examples require to drive either a base or a gate electrode of the large transistor in chip size with a constant current, it is necessary to enlarge the constant current value for accelerating the driving speed. Accordingly, there has been a problem that an increase in dissipated current obstructs a reduction in power consumption.